Pumpkin pie craters on Mercury are solar system first

The planet Mercury used these ingredients to bake several strange crater pies, complete with wrinkled crusts and cracked fillings. Spotted by a NASA probe, the craters are unlike anything seen on other rocky worlds, adding to the diversity of geologic processes known to occur in the solar system.

The pies were found in Mercury’s northern highlands, which were flooded repeatedly with volcanic eruptions early in the planet’s history. These floods buried underlying impact craters, but as the lava cooled, the crater rims became visible as the material above them wrinkled and split, forming the edges of the pie crust.

The interiors of the craters are criss-crossed with cracks called graben, which form when rock is stretched horizontally. These cracks reminded scientists of the fissures that can form as a pumpkin pie cools.

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“I haven’t really done the tectonics of the pie,” admits Tom Watters of the Smithsonian Institution in Washington, DC. “But there are a number of similarities.”

Images from Messenger now reveal the pie-like craters throughout the northern volcanic plains, which cover about 6 per cent of the planet’s surface.

“We’ve never seen anything like that on Mercury, and we’ve never really seen anything like that on any of the other terrestrial planets,” Watters says. “What is happening here? Something definitely different is going on on Mercury that hasn’t happened on the moon or Mars.”

Deep dish

Watters and colleagues modelled the formation of the craters and concluded that the cracks could have formed only in these pies, where lava arrived in quick bursts and then pooled deep enough to act just like pie filling.

“It’s the same process,” Watters says. “It’s cooling and contracting, and you’re getting some resistance to that contraction because the pie filling is in contact with the edges of the pie crust, and that crust is resisting its contraction.” By contrast, Mars and the moon’s volcanic flooding happened in thin gushes over the course of hundreds of millions of years, limiting their baking skills.

“I think it’s another indication of just how unusual Mercury is among the terrestrial planets,” Watters says. “The tectonics of Mercury may be as unique in our solar system as plate tectonics is on Earth.”